Why it plots more than just one point and how do I get rid of them

function xp=h2(t,x)

xp=x;

xp(1)=(x(2)) .^2 - 70.*x(2);

xp(2)=(x(1)) .^2 +134.*x(1) - 64 .*x(2);

hold on

[t,x]=ode23('h2', [0,100], [0,0]);

plot(x(:,1), x(:,2), 'kx')

[t,x]=ode23('h2', [0,100], [-64,70]);

plot(x(:,1),x(:,2), 'bx')

[t,x]=ode23('h2', [0,100], [-134,0]);

plot(x(:,1), x(:,2), 'kx')

[t,x]=ode23('h2', [0,100], [55.4/2,70]);

plot(x(:,1), x(:,2), 'kx')

[t,x]=ode23('h2', [0,100], [-323.4/2,70]);

plot(x(:,1), x(:,2), 'kx')

When I plot [-64,70] (stable node) and [55.4/2,70] (saddle), it doesn't appear just one 'x' on the given coord. but it plots a spiral of 'x's starting from those points to [0,0] (focus) point. How do I make the spiral disappear, and have only one 'x' point for each coord?

Answer 1

[Star Strider]

The code plots more than one point because you told it to.

Revised code:

function xp=h2(t,x)
xp=zeros(2,1);
xp(1)=(x(2)) .^2 - 70.*x(2);
xp(2)=(x(1)) .^2 +134.*x(1) - 64 .*x(2);
end

hold on
[t,x]=ode23(@h2, [0,100], [0,0]);
plot(x(:,1), x(:,2), 'kx')
[t,x]=ode23(@h2, [0,100], [-64,70]);
plot(x(:,1),x(:,2), 'bx')
[t,x]=ode23(@h2, [0,100], [-134,0]);
plot(x(:,1), x(:,2), 'kx')
[t,x]=ode23(@h2, [0,100], [55.4/2,70]);
plot(x(:,1), x(:,2), 'kx')
[t,x]=ode23(@h2, [0,100], [-323.4/2,70]);
plot(x(:,1), x(:,2), 'kx')
hold off

What result do you want?

[Stefania Maria]

It still doesn't work.

I only need five points on the graph

[Star Strider]

Replace ‘[0,100]’ for the ‘tspan’ argument with:

tv = linspace(0, 100, 5);

Then use that in every ode45 call:

hold on
[t,x]=ode23(@h2, tv, [0,0]);
plot(x(:,1), x(:,2), 'kx')
[t,x]=ode23(@h2, tv, [-64,70]);
plot(x(:,1),x(:,2), 'bx')
[t,x]=ode23(@h2, tv, [-134,0]);
plot(x(:,1), x(:,2), 'kx')
[t,x]=ode23(@h2, tv, [55.4/2,70]);
plot(x(:,1), x(:,2), 'kx')
[t,x]=ode23(@h2, tv, [-323.4/2,70]);
plot(x(:,1), x(:,2), 'kx')
hold off

That will produce five points for every ode45 call.

If you only want five points total, you need to choose which points you want to plot.

That might require:

hold on
[t,x1]=ode23(@h2, tv, [0,0]);
plot(x1(end,1), x1(end,2), 'kx')
[t,x2]=ode23(@h2, tv, [-64,70]);
plot(x2(end,1),x2(end,2), 'bx')
[t,x3]=ode23(@h2, tv, [-134,0]);
plot(x3(end,1), x3(end,2), 'kx')
[t,x4]=ode23(@h2, tv, [55.4/2,70]);
plot(x4(end,1), x4(end,2), 'kx')
[t,x5]=ode23(@h2, tv, [-323.4/2,70]);
plot(x5(end,1), x5(end,2), 'kx')
hold off

Or something similar.

Experiment to get the result you want.